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**In Chan Song, Ph.D. Seoul National University Hospital**

Propeller MRI In Chan Song, Ph.D. Seoul National University Hospital

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**Contents: Propeller sequence**

(Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) Motion artifact Theoretical basis Applications

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**Motion Cause Periodic: cardiac motion, respiration, blood flow**

Sporadic: irritable patients’ motion Translation, rotation, through-plane Artifact in MRI blurring and ghosting Cause Longer encoding step

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Scan time= TR x matrix x Average Long scan time

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**MR image reconstruction**

under the assumption of object’s motion-free condition during whole k space coverage

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**Motion artifacts -Most ubiquitous and noticeable artifacts in MRI**

due to voluntary and involuntary movement, and flow (blood, CSF) -Mostly occur along the phase encode direction, since adjacent lines of phase-encoded protons are separated by a TR interval that can last 3,000 msec or longer -Slight motion can cause a change in the recorded phase variation across the FOV throughout the MR acquisition sequence

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**Motion artifact: ghost and blurring**

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**Solution for motion compensation**

-Navigator echo usage to estimate the motion or motion related phase from extra collected data -Cardiac and respiratory gating -Respiratory ordering of the phase encoding projections based on location in respiratory cycle -Signal averaging to reduce artifacts of random motion -Short TE spin echo sequences (limited to spin density, T1-weighted scans). Long TE scans (T2 weighting) are more susceptible to motion

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**Motion (abrupt) phase error position error**

Solution Phase information Navigation Motion correction by phase information

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**Key ideas in propeller sequence**

K space: partial covering for whole image Motion detection: blade usage Correction: FFT properties’ usage

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**Diagram of the PROPELLER collection reconstruction process for motion corrected MRI.**

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Data acquisition Propeller filling Rectangular filling kx ky

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Phase Correct Redundant data must agree, remove phase from each blade image Imperfect gradient balancing, Eddy current effect: echo center shift

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James G. Pipe

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Windowing Before After Phase correction

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**Bulk Transformation Correction**

Fourier transform correspondence Image space k space Translation Phase roll Rotation Rotation Separate estimation of rotation and translation

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**rotate imagerotate data**

Fourier Transform Properties rotate imagerotate data

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**Rotation correction (magnitude image)**

Reference (only inner circle) Magnitude of the average of strips Rotation (only inner circle) Correlation

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**Blade by blade operation**

Rotation at maximum correlation Correction

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**Fourier Transform Properties**

shift image phase roll across data b is blade image, r is reference image

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max at Dx

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Translation Complex average k-space data Reference (only inner circle) Complex of the average of strips Multiplication Inverse FT (maximum)

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**Blade by blade operation**

Translation at maximum correlation Correction

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Blade Correlation throw out bad – or difficult to interpolate - data

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Through-plane motion :low weighting coeff.

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Reconstruction (FFT) non-Cartesian sampling requires gridding convolution Kx Ky

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w/motion correction

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no correction correlation correction only motion correction only full corrections

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**Artifact reduction due to head motion**

T2-FSE T2-Propeller T2-Propeller(corrected)

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DWI-EPI B=1000s/mm2 DWI-Propeller (FSE) James G Pipe, 2002

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DWI (b=700s/mm2) a. EPI (TR/TE/avg=2700/113/15) b. Propeller EPI (TR/TE/blade=1600/70/26) Wang FN, 2005

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**Useful application in propeller sequence**

Motion- or Bo-inhomogeneities – insensitive Irritable patient Diffusion weighted image Limitations in propeller sequence Redundant acquisition Long scan time: High SAR: problem in higher field MR system Solutions Undersampling (Konstantinos Arfanakis, 2005) Parallel imaging Turbopropeller (James G Pipe, 2006) Propeller EPI

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Propeller sequence Low sensitivity to image artifacts, Bo inhomogeneity and motion T2-, Diffusion-weighted images (High SNR, low geometric distortion, low SAR)

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**References 1. Pipe J, MRM 42(5): 963-62,1999.**

2. Pipe J, et al., MRM 47(1): 42-53,2002 3. Wu Y, Field AS, Alexander AL. ISMRM, Toronto, Canada, 4. Roberts TP, Haider M. ISMRM, Kyoto, Japan, 5. Sussman MS, White LM, Roberts TP. ISMRM, Kyoto, Japan, 6. Pipe J and Zwart N. Magn Reson Med 55:380–385, 2006. 7. Cheryaukaa AB, et al. Magnetic Resonance Imaging 22: , 2004

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